The Hand of the Light are referred to as Questioners because when a darkfriend is found (or suspected of being found) they are handed over to the Questioners and they are "put to the question". In other words they are tortured into revealing their secrets, admitting they are darkfriends and repenting their crimes. However, many believe that innocents are interrogated as well and are forced to admit to being a darkfriend simply to stop being tortured. It is said that once a Questioner makes up his mind about your guilt there is no changing it.
This group, along with the real world group that inspired them, The Inquisitors have lead to the development of a similar group in my worlds of Atlas and Verdenheim, called The Ministry of Light and The Holy Knights of the Creator.